The present invention relates to a cathode ray tube serving as an image display device such as a monitor tube for a terminal of office automation equipment, and in particular to a color cathode ray tube capable of displaying a high definition image.
A color cathode ray tube of this kind, a color cathode ray tube used for a monitor at a terminal of office automation equipment, for example, generally has a vacuum envelope comprised of a panel, a neck and a funnel for connecting the panel and the neck, a phosphor screen comprised of three-color phosphor picture elements coated on an inner surface of the panel, and an electron gun housed in the neck.
The electron gun for the cathode ray tube has three cathodes for generating the three electron beams in a horizontal direction, a first electrode adjacent to the cathodes, a plurality of electrodes located downstream of the first electrode and spaced in the direction of travel of the electron beams for forming a main lens. The three-color phosphor picture elements are fabricated in the form of dots or stripes and are arranged at a predetermined pitch to form the phosphor screen.
FIG. 9 is an enlarged fragmentary front view of a central portion of the phosphor screen of the above-explained color cathode ray tube. Reference numeral 90 denotes dot-shaped phosphor picture elements arranged at a predetermined pitch over the entire inner surface of the panel to form the phosphor screen. Reference numerals 91, 92 and 93 denote red (R) phosphor picture elements, green (G) phosphor picture elements and blue (B) phosphor elements, respectively. A dimension P is a pitch in a horizontal direction of an array of phosphor picture elements of a same color (a horizontal center-to-center distance between a first phosphor picture element of a first color in a first horizontal row of the array of the phosphor picture elements and a second phosphor picture element of the first color which is nearest to the first phosphor picture element of the first color and is in a second horizontal row adjacent to the first horizontal row). A dimension V is a pitch in a vertical direction of the array of the phosphor picture elements of the same color (a vertical center-to-center distance between the phosphor dots in the first and second horizontal rows of the array of phosphor picture elements, respectively).
For a color cathode ray tube employing such a phosphor screen to resolve the fine structure of a display image, it is necessary to reduce the pitches of the array of the phosphor picture elements 90 and thereby to increase the density of the phosphor picture elements 90 in the phosphor screen, and it is especially important to reduce the horizontal pitch P of the array of the phosphor picture elements 90. This is because the three electron beams emitted from the electron gun are arranged in a horizontal direction and consequently, a trio of three phosphor elements of different colors are necessarily arranged in a horizontal direction with respect to an electron beam aperture in a color selection structure, a shadow mask, for example.
This structure imposes a restriction on reduction of the pitches.
The density of the phosphor picture elements 90 is defined in terms of the number N of trios of phosphor elements of three different colors horizontally arranged in two adjacent horizontal rows, as illustrated as the number N of trios comprising . . . an (nxe2x88x921)st trio, an nth trio, an (n+1)st trio, . . . in FIG. 9.
On the other hand, in order to improve the resolution, it is also necessary to reduce a diameter of an electron beam spot produced by the electron beam striking the phosphor screen as well as the pitches of the array of the phosphor picture elements in the phosphor screen, so that picture detail contained in signals is delineated on the phosphor screen.
To meet such a demand, the conventional horizontal pitch P of about 0.3 mm of the array of the phosphor picture elements is reduced to about 0.22 mm to about 0.24 mm recently and consequently, the number of picture elements capable of being displayed is increased dramatically. Improvements of performance of electron guns reduced the diameter of the electron beam spots from about 0.7 mm to about 0.5 mm.
Especially color cathode ray tubes used for a monitor at a terminal of office automation equipment has been making progress in high information content display with reduction of the pitches of the array of the phosphor picture elements and reduction of the spot diameter produced by the electron beam, as disclosed by the Hitachi Hyoron, vol. 78, December, 1996, for example.
The color cathode ray tubes used for a monitor at a terminal of office automation equipment or the like must be capable of displaying images of various pixel densities in accordance with deflection frequency or changes of signal formats. The display area is often held constant regardless of the number of pixels forming one picture. Therefore, more is caused by interference between the pitches of the array of the phosphor picture elements and pitches of picture detail contained in signals (video signal frequencies), depending upon the numbers of pixels in vertical and horizontal directions, respectively, and the number of scanning lines, because of the fixed pitches of the array of the phosphor picture elements, and consequently, sharp images were not obtained.
For example, consider that 1300 to 1500 dots are displayed in a horizontal direction on a phosphor screen of a prior art color cathode ray tube having a size of 400 mm in the horizontal direction and about 500 mm in diagonal. In this case the modulation transfer function (the luminance response to an input sine wave signal, hereinafter MTF) is calculated to be approximately equal to or less than 10%.
It is preferable that the MTF response is at least 10% for resolving and delineating detail of letters, characters or patterns on the phosphor screen.
Consequently, it was difficult to display the standard number of display dots of 1600 to 1800 or more required for a display monitor of the above size.
Incidentally, the largest number of resolvable display dots in a horizontal direction divided by a horizontal dimension W of the phosphor screen is 3.25 to 3.75 in the above case, and even the best value for conventional cathode ray tubes is 3.9.
Here, the number of the display dots in the horizontal direction is defined in terms of the number N of trios of phosphor elements of three different colors horizontally arranged in two adjacent horizontal rows, as illustrated as the number N of trios comprising . . . an (nxe2x88x921)st trio, an nth trio, an (n+1)st trio, . . . in FIG. 9.
It is an object of the present invention to provide a color cathode ray tube capable of providing a sharp image free from occurrence of moire and having resolution sufficiently high to secure the required number of display dots, by eliminating the above problems with the conventional technique.
To accomplish the above objects, in accordance with an embodiment of the present invention, there is provided a color cathode ray tube comprising: a vacuum envelope including a panel, a neck and a funnel for connecting the panel and said neck, an electron gun housed in the neck and including a cathode structure for emitting three electron beams, a first electrode closely spaced from the cathode structure and serving as a control electrode, a second electrode serving as an accelerating electrode and a plurality of focus electrodes and an anode arranged in spaced relationship in a direction of travel of the three electron beams in the order named, a phosphor screen formed on an inner surface of the panel and composed of repeating patterns of three-color phosphor elements facing the electron gun, a color selection electrode positioned in the vacuum envelope between the electron gun and the phosphor screen, and a deflection yoke mounted around the vacuum envelope for scanning the three electron beams on the phosphor screen; the following inequalities being satisfied. {(L+1360xc3x97Dxe2x88x92600)/280}2+{(Pxe2x88x920.16)/0.06}2xe2x89xa61, L+1360xc3x97Dxe2x89xa7600, and Pxe2x89xa70.16, where D (mm) is a horizontal diameter of electron beam apertures in the first electrode, L (mm) is a distance from a midplane between the anode and one of the plurality of the focus electrodes adjacent to, but spaced from the anode, to a center of the phosphor screen, and P (mm) is a horizontal center-to-center distance between a first phosphor element of a first color of the three-color phosphor elements in a first horizontal row of the repeating patterns and a second phosphor element of the first color which is nearest to the first phosphor element and is in a second horizontal row adjacent to the first horizontal row, at the center of the phosphor screen.